In a system to which the invention relates, common control circuit wiring is employed to carry command signals, actuating signals and current at a level sufficient to effect actuation of switching relays. This approach minimizes the amount of wiring required for a given system installation but it does impose a requirement for voltage regulation in order that reliable operation will occur with changing circuit conditions, including changes to the size of the installation. The inclusion of a voltage regulated, direct current power supply as such does not present any difficulties, but a problem occurs when attempting to modulate the otherwise constant control circuit voltage with command and/or actuating signals. That is, a conventional regulated power supply will present zero impedance to an alternating signal and, thus, effectively absorb the command and actuating signals.
One attempt has been made to avoid this problem by arranging for the response time of voltage regulation to be slow relative to the frequency of modulation used by both the command and the actuating signals. However, this solution requires the introduction of R.C time constant circuit components, resulting in consequential power loss and imposing limits on the number of load devices that may be connected to a given installation.